Asbestos-cement structural sheet



Dec. 21, 1965 N. s. GREINER ETAL 3,224,205

ASBESTOS-CEMENT STRUCTURAL SHEET Filed Aug. 29, 1962 2 Sheets-Sheet 1 PRIOR ART INVENTOR. NORMAN 5.6mm. BYCl-AYTON V. Fnzucu ATTORNEY Dec. 21, 1965 N. s. GREINER ETAL 3,224,205

ASBESTOS-CEMENT STRUCTURAL SHEET Filed Aug. 29, 1962 2 Sheets-Sheet 2 60% FLANK COVERAGE ANGENT ANG E, DEGREES n 45 5O 55' 6O 65 7O '75 80 HmlM $0 1.5 'lvauh 83d m'smnqow uoLLaas INV NTOR NORMAN S. REINER YCLAYTON V Faun.

ATTORNEY United States Patent 3,224,205 ASBESTOS-CEMENT STRUCTURAL SHEET Norman S. Greiner and Clayton V. French, Somerville, N.J., assignors to Johns-Manville Corporation, New York, N.Y., a corporation of New York Filed Aug. 29, 1962, Ser. No. 220,210 7 Claims. (Cl. 61--59) This invention relates to asbestos-cement structural sheets, and more particularly, to an improved corrugated asbestos-cement structural sheet adapted for use in bulkhead constructions for inland waterways.

Rather recently, land reclamation operations have been very active in certain coastal parts of the United States. These operations often include the construction of canals filled with about four feet of sea water so that property owners can travel by boat directly from a bay or other body of water to their lots. To prevent the erosion or caving in of canal banks, they are required to be lined with bulkheads, which are commonly formed of corrugated asbestos-cement sheets joined together to form a rigid wall. The bottom portions of the sheets are generally embedded in concrete or sunk into the canal bed by a jetting process, whereby suitable holes or ditches to receive the bottom portions of the sheets are formed in the canal bed by the action of a powerful jet of water. The sheets are often provided with a concrete cap after installation and anchored by deadmen to form the finished bulkhead. Asbestos-cement sheets are Well suited for use in bulkhead constructions since they are able to resist the attack of minerals and salts found in sea Water which are corrosive to many other construction materials.

Because the lateral forces exerted by the canal bank in many instances are large, the sheets should have a maximum bending moment under saturated conditions of at least about 9,000 inch-pounds per lineal foot of Width of sheet, where the width of a sheet runs transverse to the longitudinal corrugations, and preferably a bending moment of 11,000 or 12,000 inch pounds per lineal foot of Width. Prior to the present invention, all of the known corrugated asbestos=cement sheets employed in bulkheads were of the basic standard size, having a pitch of 4.2 inches, where pitch is the distance between the peaks of successive crowns or valleys of the corrugations, and a net height of slightly more than one inch, where net height is the transverse distance between the centerlines of adjacent crown and valley portions. The bending moment of saturated standard size corrugated sheets formed of asbestos-cement compositions having a modulus of rupture of about 3,000 p.s.i. and having a standard thickness of about 0.4 inch, is about 3,000 inch-pounds per lineal foot of width, which is only about one-third of the minimum required bending moment. Attempts have been made to strengthen the basic standard size corrugated sheet by increasing the thickness to 0.6 inch or more and/or forming the sheets from compositions of high strength formulations having a modulus of rupture of about 6,000 p.s.i. While these efforts have resulted in corrugated asbestos-cement sheets of greater strength, the sheets have not been economically desirable because of the added cost involved in providing more material or more costly asbestos-cement compositions.

The deficiencies of standard size corrugated asbestoscement sheets in bulkhead installations suggest the desirability of an asbestos-cement sheet of a different contour or configuration which would impart suflicient strength to the sheet to withstand the stresses and loads encountered during use and yet not require an excessive amount of material or a more costly enriched asbestos-cement composition. Because of the high strength and pleasing appearance provided by corrugated sheets, however, it is preferred that the sheet retain a corrugated configuration.

In the design of a corrugated sheet, in addition to its strength requirements, a prime consideration is the nesting ability of stacked sheets, since for purposes of economy and efliciency, the sheets should be stacked as closely as possible during storage and shipment. The thickness of the standard size corrugated sheet is varied such that the radii of curvature of the outer surfaces of the crowns and valleys are equal to the radii of curvature of the inner surfaces of the crowns and valleys. This ensures that the sheets can be stacked with the corrugations of each sheet interfitting and mating with the corrugations of adjacent superposed sheets. The varying thickness of the standard size sheet is generally provided by forming an asbestos-cement sheet, corrugating the uncured sheet by means of corrugating rolls or other suitable apparatus, and pressing the corrugated sheet in a specially designed press adapted to impart the desired variable thickness to the crowns and valleys. To provide a corrugated asbestoscement sheet having a contour diflerent from that of the standard size sheet for purposes of strength but with a variable thickness for nesting purposes, would require more material than a sheet of uniform thickness and would necessitate a specially designed press, adding to the cost of the final product. If the corrugated sheets are provided with a uniform thickness, however, they can be produced at a faster rate and more economically, since this eliminates the pressing operation.

It is an object of the present invention to provide a corrugated asbestos-cement sheet of uniform thickness having a maximum bending moment of at least 9,000 inch-pounds per lineal foot of width.

Another object of the invention is to provide a corrugated asbestos-cement sheet of uniform thickness having a maximum bending moment of at least 9,000 inchpounds per lineal foot of width and formed of a composition having a relatively low modulus of rupture, thereby permitting use of less expensive formulations without sacrificing the strength of the final sheet product.

A further object of the invention is to provide a corrugated asbestos-cement sheet of uniform thickness having excellent nesting properties.

Another object of the invention is to provide a corrugated asbestos-cement sheet having relatively deep corrugations and adapted for use in a bulkhead construction.

Briefly, the present invention comprises a corrugated asbestos-cement sheet comprising a plurality of arcuate crown and valley portions connected by straight flank portions, the sheet being of uniform thickness, having a maximum bending moment of at least 9,000 inch-pounds per lineal foot of width, and having a section modulus of at least about 3.33 cubic inches per lineal foot of width. The section modulus per lineal foot of Width is arrived at by multiplying the section modulus of one corrugation (cubic inches per corrugation) by the number of corrugations in one foot, which is the quotient of 12 (inches per foot) divided by the pitch (inches per corrugation). Thus, when the pitch is less than one foot the section modulus per lineal foot of width is greater than the section modulus of a single corrugation. The mean radius of curvature of the crowns and valleys is at least about 1.75 inches, and each corrugation has a pitch and net height not exceeding about 11 inches and 5 inches, respectively. The flank portions are at least 1.30 inches in length, and the over-all design is such that at least 60% of adjacent flank portions of adjacent sheets are engaged when the sheets are in nested relation. Such a sheet provides more than three times the strength but weighs less per ice square foot than the standard size corrugated asbestoscement sheet. In addition, the sheet presents a more pleasing appearance due to the relatively deep corrugations thereof and has greater efiiciency than the standard size corrugated sheet.

The nature of the invention will be more fully understood and other objects may become apparent, when the following detailed description is considered in connection with the accompanying drawing, wherein:

FIG. 1 is an end elevational view of a corrugation of a standard size corrugated asbestos-cement sheet of the prior art;

FIG. 2 is an end elevational view of a corrugation of the asbestos-cement sheet of the present invention;

FIG. 3 is a pictorial representation of a plurality of corrugated sheets of the present invention joined together to form a continuous wall;

FIG. 4 is an end elevational view of two super-posed corrugated sheets of the present invention, illustrating the nesting characteristics thereof; and

FIG. 5 is a graphical representation of the section modulus of asbestos-cement corrugated sheets at various tangent angles, and capable of 60% flank engagement when nested with a similar sheet.

Referring to FIG. 1 of the drawing, a standard size corrugation of the prior art is indicated at 10. The pitch p, which is the distance between the peaks of successive crowns 12 or valleys 14, is 4.2 inches, and the net height h, which is the distance between the center lines of adjacent crowns and valleys, is 1.05 inches. The thickness of the corrugation at points between the crowns 12 and valleys 14 is 0.4 inch, while the thicknesses of the crowns and valleys are slightly greater than 0.4 inch to provide for the radius of curvature of the the outer surface of a crown or valley to be equal to the radius of curvature of the inner surface of a crown or valley. This ensures that the sheets will be closely nested when stacked so that a minimum of space is required for storage or shipment. The physical properties of a standard size corrugated sheet having a saturated modulus of rupture of about 3,000 p.s.i. are as follows:

Section modulus (cubic inches per lineal foot of of width) 1.04 Dry weight (p.s.f.) 4.0 Saturated weight (p.s.f.) 4.8 Dry density (p.c.f.) 112 Ultimate uniform load bearing ability, 5 ft. span,

Saturated (p.s.f.) Efiiciency, 5 ft. span (ultimate uniform load bearability divided by saturated weight) 17 The standard size corrugated sheet is formed by corrugating an uncured asbestos-cement sheet by suitable means such as corrugating rolls or a corrugated press. The initially formed corrugated sheet is then exposed to a press operation which provides the crowns and valleys of the sheet with the desired varying thickeness.

Since the maximum bending moment of a corrugated sheet is equal to the modulus of rupture multiplied by the section modulus, it can be seen that even with a modulus of rupture of about 3,000 p.s.i., the maximum bending moment of the standard size sheet would be only slightly more than 3,000 inch-pounds per lineal foot of width. In order to increase the maximum bending moment without changing the contour of the corrugations, it is necessary either to increase the thickness of the sheet, raise the density, or utilize a more costly high strength asbestos-cement composition. While all of these modifications do increase the strength of the sheet somewhat, they also increase the cost of manufacture and result in a relatively heavy product which is more difficult to handle.

Referring to FIG. 2, a single corrugation of the preferred embodiment of the sheet of the present invention is indicated at 16. The pitch P of a corrugation is 9.4 inches, the net height H is 4.56 inches, and the uniform thickness t is 0.30 inch. The mean radius of curvature OA of a crown 18 of the corrugation is 2.06 inches, and the mean radius of curvature CE of a valley 20 is 2.00 inches, where the mean radius of curvature is measured from the center of curvature to the centerline of the sheet. The tangent angle AOB, which is the angle between the crest of a corrugation and the point of tangency between the crown and the straight flank portion ED, is equal to 65. The tangent angle ECD, which is the angle between the lowermost point of a valley and the point of tangency between the valley and the straight flank portion ED is also equal to 65. The length of the straight flank portion ED is 2.45 inches, and the centers of curvature O and C are spaced on opposite sides of the neutral axis of the corrugation, by a distance of 0.25 inch. The radius of curvature 0A is slightly greater than the radius of curvature CE to provide for the straight flank portion BD to be tangent to the arcs BA and DE.

As shown in FIG. 3, the sheet 16 is preferably three and one-half corrugations wide, terminating in partial corrugations 22 and 24. The partial corrugation 24 has a net height which is less than the net height of the other corrugations by an amount equal to the thickness t of the sheet, which permits the partial corrugation 22 of one sheet to overlap the partial corrugation 24 of an adjacent sheet to present the over-all appearance of a corrugated wall of unitary construction. Because of this arrangement, the tangent angle of the interior crown and valley portions, that is, those crown and valley portions not located at the ends of the sheet, may be different than the tangent angle of crown and valley portions at the end of the sheet. Adjacent sheets may be fastened together at their overlapping joints by means of tie wires or other suitable fasteners, and the joints may be suitably sealed to make them watertight. The specific features of the joint structure are not shown since they are well known and constitute no part of the present invention.

The physical properties of the corrugated asbestoscement sheets of the present invention, dimensioned according to the foregoing disclosure, are as follows:

Section modulus (cubic inches per lineal foot of width) 5.52 Dry weight (p.s.f.) 3.60 Saturated weight (p.s.f.) 4.5 Dry density (p.c.f.) Ultimate uniform load bearing ability, 5 ft. span,

saturated (p.s.f.) 250 Efliciency, 5 ft. span (ultimate uniform load bearing ability divided by saturated weight) 55 As previously stated, to determine the maximum bending moment of a sheet, the section modulus is multiplied by the modulus of rupture. It readily can be determined that a bending moment of 9,000 inch-pounds per lineal foot of width of the preferred embodiment of the present invention can be achieved by utilizing an asbestos-cement composition having a modulus of rupture of as low as 1,630 p.s.i., although as will be pointed out hereinafter, it is preferred to utilize a composition having a greater modulus of rupture. The high strength characteristics of the sheet are further emphasized when the bending moment is computed using a composition having a modulus of rupture of 2,500 p.s.i., which results in a bending moment of 13,800 inch-pounds per lineal foot of width, more than four times the value of the maximum bending moment of a standard size corrugated sheet formed of a composition having a modulus of rupture of about 3,000 p.s.r.

Since the weight per square foot of the sheet of the present invention is less than the weight per square foot of the standard size corrugated sheet, the greater strength of the first mentioned sheet is even more remarkable. This is clearly brought out by a comparison of the chi- .5 ciencies of the sheets, which reveals that the sheet of the present invention has an, efficiency of more than three times that of the standard size sheet.

Referring now to FIG. 4, two corrugated sheets 16 are shown in superposed relation as they would appear when stacked for storage or shipment. The gap 28 between adjacent crowns 18 and valleys 20 of the sheets is held to a minimum due to the design of the corrugations which enables about 70%80%, according to manufacturing tolerances, of adjacent straight flank surfaces 30 to be engaged. The percentage of straight flank portions of adjacent stacked sheets which are engaged is important, as is explained more fully hereinafter.

The corrugated sheets are manufactured by forming a sheet of an asbestos-cement composition, corrugating the uncured sheet by a molding operation, and curing the corrugated sheet by a normal cure or steam cure. There is a tendency for tiny or incipient cracks to form in the corrugations during the molding operation when the radius of curvature of the crowns and valleys is too small, which cracks are undesirable because they reduce the transverse strength of the corrugations. It has been found that in manufacturing a corrugated asbestos-cement sheet of approximately 0.3 inch thickness, the formation of cracks is prevented if the mean radius of curvature is no smaller than approximately 1.75 inches. A mean radius of curvature of two inches is preferred to provide a safe margin to ensure against the formation of cracks. In addition to the radius of curvature, the particular composition from which the sheets are formed has an effect on the ability of the sheet to be molded on a small radius. A typical basic composition for a normal cured product comprises about 45% by weight of asbestos fibers and about 55% by weight of hydraulic cement, where the asbestos fibers have a length corresponding to that denoted by 6D of the Canadian Asbestos Fiber Classification System. While this composition can be used with good results to mold a corrugated sheet of approximately 0.3 inch uniform thickness and having a radius of curvature of two inches, a composition including relatively longer fibers provides even better results insofar as ensuring against formation of cracks is concerned. An example of such formulation is as follows:

Percent R Asbestos fiber 20 6D Asbestos fiber 20 Hydraulic cement 50 Cementitious scrap 10 A typical formulation which may be used where the product is to be steam cured is as follows:

Percent 5R Asbestos fiber 20 6D Asbestos fiber 20 Hydraulic cement 38 Silica flour 22 The above-described corrugated sheet represents the preferred embodiment of the invention. The dimensions indicated fall within the limits imposed by the requirements of a sheet having a maximum bending moment of at least 9,000 inch-pounds per lineal foot of width, a uniform thickness to permit its formation by a molding process, a radius of curvature of the crowns and valleys to permit molding of the corrugations without forming cracks in the curved portions, and good nesting properties.

As previously indicated, the modulus of rupture of the asbestos-cement composition may be varied to vary the bending moment of the sheet, since the bending moment is equal to the modulus of rupture multiplied by the section modulus. In practice, it is not desirable to use a composition with a very low modulus of rupture, even if the section modulus is of suflicient value to theoretically provide for the desired bending moment, because sheets composed of such compositions tend to flake or crumble, and generally to exhibit poor overall handling properties.

It is desirable also to limit the maximum value of the modulus of rupture of an asbestos-cement composition because of the cost involved in providing a rich composition having a high modulus of rupture. For economical reasons, therefore, the composition should have a modulus of rupture of no more than about 2,700 p.s.i., and preferably 2,500 p.s.i. It should be understood that these values relate to the saturated modulus of rupture since the sheets are designed primarily for use in bulkhead constructions.

As previously noted, to prevent the formationof cracks in the curved portions of molded corrugated sheets, the mean radius of curvature of the crowns and valleys should be at least 1.75 inches for a sheet approximately 0.30 inch thick, and preferably about two inches, to provide an adequate margin of safety. If a greater sheet thickness is provided, the minimum mean radius of curvature required to prevent crack formation during molding is greater than 1.75 inches, and conversely, if a lesser sheet thickness is provided, the minimum radius of curvature required is less than 1.75 inches. The preferred thickness of about 0.30 inch permits economical manufacture of a corrugated sheet proportioned to provide a pleasing appearance, and adequate strength. A greater thickness would result in more costly sheets difiicult to handle by workmen, while a lesser thickness would reduce the strength of the sheet. For example, in the preferred embodiment of the present invention, a reduction in sheet thickness from 0.30 to 0.20 inch reduces the section modulus from 5.52 to about 3.70 cubic inches per lineal foot of width. Since it is not feasible to hold the manufacturing tolerance to exacting limits, the preferred range of thickness is about 0.25 inch to about 0.35 inch, although the thickness may be outside this range so long as the strength requirements are maintained, and the limits of other dimensions, as discussed hereinafter, are observed.

if corrugated asbestos-cement sheets are formed with arcuate crowns and valleys with no connecting straight flank portions, the sheets cannot be stacked practicably because they will be in line contact, which results in a concentration of pressure at these lines, causing failure or cracking of the sheets. By providing a straight flank portion tangent to and connecting adjacent crowns and valleys, superimposed sheets are in surface contact rather than line contact. However, it has been found that unless adjacent stacked sheets have at least 60% flank engagement, the surface contact will be concentrated on such a small area that cracking might occur.

With a fixed radius of curvature it is possible to vary the tangent angle and the length of flank portion to obtain virtually any desired percentage of flank engagement between adjacent nested sheets. There are limits, however, between which the tangent angle should be maintained. If the tangent angle is small, the percentage of flank engagement between nested sheets is relatively great, but the section modulus of each sheet is relatively low.

If the tangent angle is large, the percentage of flank engagement between nested sheets is relatively low, but the section modulus of each sheet is relatively high. As previously stated, the maximum desired modulus of rupture is 2,700 psi; therefore, to provide the minimum bending moment of 9,000 inch-pounds per lineal foot of width, the section modulus must be at least 3.33 cubic inches per lineal foot of width. At this value of section modulus, a corrugated sheet of 0.3 inch thickness has a pitch of 8.30 inches and a net height of 3.15 inches. It has been determined that for a corrugated sheet of 0.3 inch thickness, having a mean radius of curvature of 2.0 inches and a tangent angle of 59, a straight flank portion can be provided of such length (1.30 inches) that 60% flank engagement is attained between nested sheets, resulting in about the minimum required section modulus of 3.33 cubic inches per lineal foot of width. This is illustrated in the graph of FIG. which shows the section moduli that can be attained for corrugated sheets having various tangent angles and straight flank portions capable of 60% flank engagement when nested with similar sheets.

It is apparent that even if a very high percentage of flank engagement is sought, a relatively large tangent angle may be utilized so long as the length of the straight flank portion is increased a suflicient amount. But it is not desirable to have a flank portion of too great a length since the net height and pitch of the corrugations are increased correspondingly, thereby increasing the weight of the sheet. If the dimensions of the corrugations were too great, it would be necessary to limit the width to one or two corrugations in order to permit ready handling, which would cause the installation time to be too long; in addition, the sheet would be more expensive due to the extra material required. Another consideration is whether a suitable number of corrugations can be formed from a flat sheet of asbestos-cement of standard width, which may be 50 inches, for example. It has been determined that in order to provide a corrugated asbestos-cement sheet having the desired strength and nesting characteristics, and which is readily economically manufactured in handleable widths of pleasing appearance, the pitch should not exceed about eleven inches, and the net height should not exceed about five inches. The linear width of each corrugation should be such that a standard size asbestos-cement sheet can be molded to form a plurality of corrugations terminating in partial end corrugations to permit lapped joints, which provide a good seal and enhance the overall appearance of installed corrugated sheets.

Reviewing the various limitations on the design of corrugated asbestos-cement sheets, for a sheet having a thick ness of 0.3 inch, the minimum mean radius of curvature is about 1.75 inches, and preferably 2.00 inches to provide for a margin of safety with respect to preventing the formation of cracks during the molding process. The percentage of flank engagement of superposed sheets should be at least 60% to ensure adequate nesting without failure of the lowermost sheets due to excessive stress at the areas of contact, the preferred percentage range of flank engagement being 70% or more. The flank length may vary according to the percentage of flank engagement of superposed sheets and according to the tangent angle of the corrugations. The tangent angle may vary according to the desired percentage of flank engagement, the section modulus, and the net height and pitch of the corrugations. It has been determined for a sheet of nominal 0.3 inch thickness, that in order to provide the minimum 60% flank engagement and the minimum section modulus of 3.33 cubic inches per lineal foot of width, a minimum tangent angle of 59 and a minimum flank length of 1.30 inches may be utilized. A smaller tangent angle is permissible if the flank length is such that the minimum section modulus is provided. This would result in a greater percentage of flank engagement and greater net height and pitch, which latter two dimensions should be maintained under 5 inches and 11 inches, respectively, for purposes of maufacture, economy, and handleability. The use of a greater tangent angle than 60 results in a requirement for greater flank length and section modulus in order to provide the minimum 60% flank engagement, and is limited by the upper limits of the net height and pitch. The modulus of rupture of the asbestos-cement composition should be within the range of 1,8002,700 p.s.i. and preferably within the range of 2,000-2,500 p.s.i.

The values of the section modulus and radius of curvature are based on a uniform sheet thickness of 0.3 inch, which, in the preferred embodiment, res l ed n a Section modulus of 5.52 cubic inches per lineal foot of sheet width. By increasing the sheet thickness to 0.4 inch, the section modulus is increased to about 7.08 cubic inches per lineal foot of width, while by decreasing the thickness to 0.2 inch, the section modulus is decreased to about 3.70 cubic inches per lineal foot of width. The thickness may be varied as desired so long as the various relationships of dimensions and values of section modulus described above are maintained.

It should be understood that although the above-disclosed corrugated sheet has been described specifically in connection with its use in bulkheads for inland waterways, it may be used in any structure requiring high strength structural sheets.

It should now be apparent that the present invention provides an asbestos-cement corrugated sheet having greater strength, less weight per square foot, and greater efliciency than the well-known standard size corrugated sheet heretofore used. Although the sheet thickness is uniform throughout its length and width to permit economical manufacture by molding, superposed sheets nest excellently during storage or shipment.

It should be understood that variations and modifications of the present invention may be made without departing from the spirit of the invention. It also should be understood that the scope of the invention is not to be interpreted as limited to the specific embodiment disclosed herein, but only in accordance with the appended claims, when read in the light of the foregoing disclosure.

What we claim is:

1. A corrugated asbestos-cement sheet comprising (a) a plurality of arcuate crown and valley portions connected by tangential straight flank portions,

(b) said sheet being of uniform thickness throughout its length in the range of about 0.25 inch to about 0.35 inch,

(c) the tangent angle of at least each interior crown and valley portion being substantially similar and in the range of about 59 to about 70,

(d) the mean radius of curvature of the crown and valley portions being not less than about 1.75 inches,

(e) each corrugation having a pitch not exceeding about 11 inches and a net height not exceeding about 5 inches,

(f) said sheet having a section modulus of not less than about 3.33 inches per lineal foot of width,

(g) the fiank portion being at least about 1.30 inches in length and of such length that when said sheet and a similar sheet are nested, the adjacent flank portions thereof are engaged along at least 60% of their length, and

(h) said sheet having a maximum bending moment of at least 9,000 inch pounds per lineal foot of width.

2. A corrugated asbestos-cement sheet as recited in claim 1, wherein the density of the sheet does not exceed approximately p.c.f. and the efliciency of the sheet is at least 55%, where eificiency is determined by dividing the ultimate load bearing ability per unit area of the sheet, as measured under saturated conditions over a five foot span, by the saturated weight per unit area of the sheet.

3. A corrugated asbestos-cement sheet comprising:

(a) a plurality of arcuate crown and valley portions connected by tangential straight flank portions,

(b) the tangent angle of at least each interior crown and valley portion being substantially similar and in the range of about 59 to about 70,

(c) said sheet being of uniform thickness in the range of about 0.25 inch to about 0.35 inch,

(d) the mean radius of curvature of the crown and valley portions being in the range of about 1.75 inches to 2.25 inches,

(e) each corrugation having a pitch not exceeding about 11 inches and a net height not exceeding about 5 inches, and

(f) said sheet having a section modulus of at least about 3.33 cubic inches per lineal foot of width.

4. A corrugated asbestos-cement sheet as recited in claim 3, wherein the composition of the sheet has a modulus of rupture in the range of about 1,800 p.s.i. to about 2,700 p.s.i., and wherein the density of the sheet does not exceed approximately 100 p.c.f.

5. A structural wall comprising:

(a) a plurality of corrugated asbestos-cement sheets connected to form a continuous corrugated wall,

(b) each sheet comprising:

(1) a plurality of crown and valley portions connected by straight flank portions,

(2) the tangent angle of at least each interior crown and valley portions being substantially similar and in the range of about 59 to about 70, and the straight flank portion being at least about 1.30 inches in length,

(3) the mean radius of curvature of the crown and valley portions being in the range of about 1.75 inches to 2.25 inches,

(4) each corrugation having a pitch of not more than about 11 inches and a net height of not more than about 5 inches,

(5) said sheet having a uniform thickness in the range of about 0.25 inch to about 0.35 inch, and

(6) the section modulus of the sheet being at least about 3.33 cubic inches per lineal foot of width.

6. A structural wall as recited in claim 5, wherein each sheet terminates in a partial corrugation, and adjacent sheets are connected with their adjacent partial corrugations overlapped.

7. A plurality of corrugated asbestos-cement structural sheets in nested relationship, each sheet comprising:

(a) a plurality of arcuate crown and valley portions connected by tangential straight flank portions,

('b) the crown and valley portions having a mean radius of curvature of at least about 1.75 inches and not exceeding about 2.25 inches, and at least the interior crown and valley portions having substantially similar tangent angles of about 59 to about 70,

(c) said sheet being of uniform thickness in the range of about 0.25 inch to about 0.35 inch,

(d) each corrugation having a pitch not exceeding about 11 inches and a net height not exceeding about 5 inches,

(e) the section modulus of the sheet being at least about 3.33 cubic inches per lineal foot of width, and

(f) the flank portions of each corrugation being at least 1.30 inches in length; and the flank portions of adjacent nested sheets being in engagement over at least 60% of their lengths.

References Cited by the Examiner FOREIGN PATENTS 625,159 8/ 1961 Canada. 228,782 2/1925 Great Britain. 355,244 8/1931 Great Britain.

OTHER REFERENCES Detchums Structural Eng. Hand Book, pub. by Mc- Graw-Hill, New York, 1924, p. 648.

EARL I. WITMER, Primary Examiner.

JACOB SHAPIRO, Examiner. 

1. A CORRUGATED ASBESTOS-CEMENT SHEET COMPRISING (A) A PLURALITY OF ARCUATE CROWN AND VALLEY PORTIONS CONNECTED BY TANGENTIAL STRAIGHT FLANK PORTIONS, (B) SAID SHEET BEING OF UNIFORM THICKNESS THROUGHOUT ITS LENGTH IN THE RANGE OF ABOUT 0.25 INCH TO ABOUT 0.35 INCH, (C) THE TANGENT ANGLE OF AT LEAST EACH INTERIOR CROWN AND VALLEY PORTION BEING SUBSTANTIALLY SIMILAR AND IN THE RANGE OF ABOUT 59* TO ABOUT 70*, (D) THE MEAN RADIUS OF CURVATURE OF THE CROWN AND VALLEY PORTIONS BEING NOT LESS THAN ABOUT 1.75 INCHES, (E) EACH CORRUGATION HAVING A PITCH NOT EXCEEDING ABOUT 11 INCHES AND A NET HEIGHT NOT EXCEEDING ABOUT 5 INCHES, (F) SAID SHEET HAVING A SECTION MODULUS OF NOT LESS THAN ABOUT 3.33 INCHES PER LINEAL FOOT OF WIDTH, (G) THE FLANK PORTIONO BEING AT LEAST ABOUT 1.30 INCHES IN LENGTH AND OF SUCH LENGTH THAT WHEN SAID SHEET AND A SIMILAR SHEET ARE NESTED, THE ADJACENT FLANK PORTIONS THEREOF ARE ENGAGED ALONG AT LEAST 60% OF THEIR LENGTH, AND (H) SAID SHEET HAVING A MAXIMUM BENDING MOEMENT OF AT LEAST 9,000 INCH POUNDS PER LINEAL FOOT OF WIDTH. 